Single Self-assembled InAs Quantum Dots Research Articles

Observation of coupling between zero- and two-dimensional semiconductor systems based on anomalous diamagnetic effects

We report the direct observation of coupling between a single self-assembled InAs quantum dot and a wetting layer, based on strong diamagnetic shifts of many-body exciton states using magneto-photoluminescence spectroscopy. An extremely large positive diamagnetic coefficient is observed when an electron in the wetting layer combines with a hole in the quantum dot; the coefficient is nearly one order of magnitude larger than that of the exciton states confined in the quantum dots. Recombination of electrons with holes in a quantum dot of the coupled system leads to an unusual negative diamagnetic effect, which is five times stronger than that in a pure quantum dot system. This effect can be attributed to the expansion of the wavefunction of remaining electrons in the wetting layer or the spread of electrons in the excited states of the quantum dot to the wetting layer after recombination. In this case, the wavefunction extent of the final states in the quantum dot plane is much larger than that of the initial states because of the absence of holes in the quantum dot to attract electrons. The properties of emitted photons that depend on the large electron wavefunction extents in the wetting layer indicate that the coupling occurs between systems of different dimensionality, which is also verified from the results obtained by applying a magnetic field in different configurations. This study paves a new way to observe hybrid states with zero- and two-dimensional structures, which could be useful for investigating the Kondo physics and implementing spin-based solid-state quantum information processing.

Terahertz intersublevel transitions in single self-assembled InAs quantum dots with variable electron numbers.

We propose a method for performing terahertz spectroscopy on nanometer (nm)-scale systems by using metal nanogap electrodes. Intersublevel transition spectra of single self-assembled InAs quantum dots (QDs) have been measured with high signal/noise ratios by using a single electron transistor geometry that consists of a QD and nanogap metal electrodes as a terahertz detector. Photocurrent distribution with respect to the Coulomb diamonds indicates that there are two mechanisms for the photocurrent generation. When the p shell was fully occupied, we observed rather simple photocurrent spectra induced by the p → d transitions. However, when the p shell was half-filled, the photocurrent spectra exhibited a markedly different behavior, which we attribute to the fluctuation in electron configuration when the empty p state is filled back from the electrodes.

Terahertz Photon-Assisted Tunneling in InAs Quantum Dots

We have investigated electron transport in a single self-assembled InAs quantum dot (QD) coupled to nanogap metal electrodes under terahertz (THz) radiation. The fabricated QD samples operated as single electron transistors in a few electron regime, exhibiting clear shell structures. Under the THz radiation, in addition to the original Coulomb oscillation peaks, new side-peaks showed up. The dependence of the new side-peak current on the THz power follows the prediction of the photon-assisted tunneling (PAT) theory. Moreover, two types of PAT processes were observed in the THz range; the ground state resonance and the photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, a very high coupling efficiency between the THz waves and the QDs was realized in our system and we observed multi-photon absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade. This high coupling efficiency between THz wave and electrons in QDs opens a way to the manipulation of single electron charge/spin states in the THz frequency range.

Large modulation of zero-dimensional electronic states in quantum dots by electric-double-layer gating

Electrical manipulation and read-out of quantum states in zero-dimensional nanostructures by nano-gap metal electrodes is expected to bring about innovation in quantum information processing. However, electrical tunability of the quantum states in zero-dimensional nanostructures is limited by the screening of gate electric fields. Here we demonstrate a new way to realize wide-range electrical modulation of quantum states of single self-assembled InAs quantum dots (QDs) with a liquid-gated electric-double-layer (EDL) transistor geometry. The efficiency of EDL gating is 6-90 times higher than that of the conventional solid gating. The quantized energy level spacing is modulated from ~15 to ~25 meV, and the electron g-factor is electrically tuned over a wide range. Such a field effect tuning can be explained by the modulation in the confinement potential of electrons in the QDs. The EDL gating on the QDs also provides potential compatibility with optical manipulation of single-electron charge/spin states.

Electrically tunable three-dimensionalg-factor anisotropy in single InAs self-assembled quantum dots

Three-dimensional anisotropy of the Lande g-factor and its electrical modulation are studied for single uncapped InAs self-assembled quantum dots (QDs). The g-factor is evaluated from measurement of inelastic cotunneling via Zeeman substates in the QD for various magnetic field directions. We find that the value and anisotropy of the g-factor depends on the type of orbital state which arises from the three-dimensional confinement anisotropy of the QD potential. Furthermore, the g-factor and its anisotropy are electrically tuned by a side-gate which modulates the confining potential.

Phase-locked indistinguishable photons with synthesized waveforms from a solid-state source

Resonance fluorescence in the Heitler regime provides access to single photons with coherence well beyond the Fourier transform limit of the transition, and holds the promise to circumvent environment-induced dephasing common to all solid-state systems. Here we demonstrate that the coherently generated single photons from a single self-assembled InAs quantum dot display mutual coherence with the excitation laser on a timescale exceeding 3 s. Exploiting this degree of mutual coherence, we synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field. In contrast to post-emission filtering, our technique avoids both photon loss and degradation of the single-photon nature for all synthesized waveforms. By engineering pulsed waveforms of single photons, we further demonstrate that separate photons generated coherently by the same laser field are fundamentally indistinguishable, lending themselves to the creation of distant entanglement through quantum interference.

Photon-Assisted Tunneling through Self-Assembled InAs Quantum Dots in the Terahertz Frequency Range

We have investigated terahertz (THz) photon-assisted tunneling in single self-assembled InAs quantum dots (QDs). Two types of photon-assisted tunneling processes have been observed in the THz range: ground state resonance and photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, we could realize a very high coupling efficiency between THz waves and QDs and observed multiphoton absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade.

The influence of temperature on the photoluminescence properties of single InAs quantum dots grown on patterned GaAs

We report the temperature-dependent photoluminescence of single site-controlled and self-assembled InAs quantum dots. We have used nanoimprint lithography for patterning GaAs(100) templates and molecular beam epitaxy for quantum dot deposition. We show that the influence of the temperature on the photoluminescence properties is similar for quantum dots on etched nanopatterns and randomly positioned quantum dots on planar surfaces. The photoluminescence properties indicate that the prepatterning does not degrade the radiative recombination rate for the site-controlled quantum dots.

Control of supercurrent in a self-assembled InAs quantum dot Josephson junction by electrical tuning of level overlaps

We study supercurrent in a single InAs self-assembled quantum dot contacted with superconducting leads and demonstrate that for regions where energy level spacing and charging energy are smaller than tunnel coupling, the supercurrent may be controlled by the degree of overlaps between energy levels, which is tunable using a side-gate electrode. In such regions, we find strong correlation between the supercurrent and the normal state conductance when the device parameters are tuned. In a Kondo regime with low Kondo temperature, we find that the scaling of the supercurrent and normal state conductance varies when the side-gate voltage is changed.

Electrically tuned spin–orbit interaction in an InAs self-assembled quantum dot

Electrical control over electron spin is a prerequisite for spintronics spin-based quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spin-orbit interaction in two-dimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spin-orbit interaction in the range 50-150 µeV while maintaining the electron occupation of a single self-assembled InAs quantum dot. We determine the spin-orbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.

Electrical control of Kondo effect and superconducting transport in a side-gated InAs quantum dot Josephson junction

We measure the non-dissipative supercurrent in a single InAs self-assembled quantum dot (QD) coupled to superconducting leads. The QD occupation is both tuned by a back-gate electrode and lateral side-gate. The geometry of the side-gate allows tuning of the QD-lead tunnel coupling in a region of constant electron number with appropriate orbital state. Using the side-gate effect we study the competition between Kondo correlations and superconducting pairing on the QD, observing a decrease in the supercurrent when the Kondo temperature is reduced below the superconducting energy gap in qualitative agreement with theoretical predictions.

Large Anisotropy of the Spin-Orbit Interaction in a Single InAs Self-Assembled Quantum Dot

The anisotropy of the spin-orbit interaction (SOI) is studied for a single uncapped InAs self-assembled quantum dot holding just a few electrons. The SOI energy is evaluated from anticrossing or SOI-induced hybridization between the ground and excited states with opposite spins. The magnetic angular dependence of the SOI energy falls on an absolute cosine function for azimuthal rotation, and a cosinelike function for tilting rotation. Furthermore, the SOI energy is quenched for a specific magnetic field vector. The angular dependence of SOI is found to compare well with calculation of Rashba SOI in a two-dimensional harmonic potential.

Kondo-enhanced Andreev transport in single self-assembled InAs quantum dots contacted with normal and superconducting leads

We study transport in self-assembled InAs quantum dots contacted with one superconducting and one normal-metal electrode. Low bias transport is dominated by Andreev processes which are sensitive to local correlations such as electron-electron interaction and the Kondo effect. We identify that, for appropriate tunnel coupling with normal and superconducting leads, Andreev transport is enhanced by the Kondo effect and that the Kondo temperature is reduced relative to the normal state due to lack of low-energy excitations with the superconducting lead.

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.

Control of tunnel coupling strength between InAs quantum dots and nanogap metallic electrodes through In-Ga intermixing

We have investigated the growth-temperature, T G, dependence of the electronic properties of single self-assembled InAs quantum dots (QDs) coupled to nanogap metallic electrodes. The orbital quantization energies of QDs and the tunnel resistances exhibited strong T G-dependence due to In-Ga intermixing during QD formation. It was found that the transparency of the tunnel junctions is controllable over a very wide range by simply changing the size and the growth temperature of QDs. By realizing strong QD-electrodes coupling, very high Kondo temperature T K∼80 K was observed in our InAs QD system.

Effect of In–Ga intermixing on the electronic states in self-assembled InAs quantum dots probed by nanogap electrodes

We have investigated the effect of In–Ga intermixing on the electronic states in single self-assembled InAs quantum dots (QDs) coupled to nanogap metallic electrodes. The orbital quantization energies of the QDs and the tunnel resistances exhibited a strong dependence on growth temperature, TG, due to In–Ga intermixing during QD formation. When the intermixing was suppressed by reducing TG to 470 °C, the electron wave functions in the QDs become more extended in space and QD-electrode coupling sufficiently strong to form the Kondo singlet states at 4.3 K was realized even in a small QD of ∼45 nm diameter.

Strong coupling through optical positioning of a quantum dot in a photonic crystal cavity

Single self-assembled InAs quantum dots embedded in GaAs photonic crystal defect cavities are a promising system for cavity quantum electrodynamics experiments and quantum information schemes. Achieving controllable coupling in these small mode volume devices is challenging due to the random nucleation locations of individual quantum dots. We have developed an all optical scheme for locating the position of single dots with sub-10 nm accuracy. Using this method, we are able to deterministically reach the strong coupling regime with a spatial positioning success rate of approximately 70%. This flexible method should be applicable to other microcavity architectures and emitter systems.

Observation of supercurrent in single InAs self-assembled quantum dots coupled to superconducting leads

We report a first direct observation of proximity supercurrent in a single InAs self-assembled quantum dot coupled to superconducting electrodes. We utilize large SAQDs with strong dot-lead coupling allowing sufficient overlap of quantized state that charge fluctuation in the dot may exceed one electron. Although the charging energy is larger than the superconductivity gap, Δ, the voltage characteristic measured in the current-driven four-terminal geometry signifies a supercurrent flow though the dot. The observed critical current is of the order of 10 pA, and much smaller than expected from theory. This current also shows the parity effect with the number of electrons in the dot, suggesting that the singlet correlation impedes the Cooper pair tunneling.

Quantitative evaluation of spin-orbit interaction in InAs quantum dots

Influence of spin-orbit interaction (SOI) on the energy spectrum is studied for single uncapped InAs self-assembled quantum dots (SAQDs) holding just a few electrons. We measure Coulomb oscillations with magnetic field, and identify lifting of spin doublets in the low field range and transitions in the ground states with keeping the total angular momentum in the high field range. Both of these features are induced by large Zeeman splitting in the present SAQDs. For the ground state transitions, anti-crossing due to the SOI hybridization between the ground and excited states with opposite spins is observed in the excitation spectra, and the SOI energy of about 0.1 meV is evaluated from the anti-crossing size. This energy is comparable to that of previous report on InAs nanowires, and reproduced by simple calculation using the g-factor also derived in the excitation spectra.

The Kondo effect observed up to TK∼80 K in self-assembled InAs quantum dots laterally coupled to nanogap electrodes

We have fabricated single electron tunneling structures by forming nanogap metallic electrodes directly upon single self-assembled InAs quantum dots (QDs). The fabricated samples exhibited clear Coulomb blockade effects. Furthermore, Kondo effect was observed when strong coupling between the electrodes and the QD was realized using a large QD with a diameter of ∼100 nm. From the temperature dependence of the linear conductance at the Kondo valley, the Kondo temperature T K was determined to be ∼81 K. This is the highest T K ever reported for artificial quantum nanostructures. The observed very high T K is due to strong QD–electrode coupling and large charging/orbital-quantization energies in the present self-assembled InAs QD structures.